Nickel plating solution

ABSTRACT

The present invention relates to a boric-acid free nickel plating composition which does not include organic carboxylic acid but has high bath-pH-stability. The nickel plating composition provides a nickel plating film suitable for the use of electronic components.

This application claims the benefit of Japanese application no. 2016-208151, filed on Oct. 24, 2016.

FIELD OF THE INVENTION

The present invention relates to a boric-acid free nickel plating composition which does not include organic carboxylic acid but has high bath pH stability. The nickel plating composition provides a nickel plating film suitable for the use of electronic materials such as under bump metal (UBM).

BACKGROUND OF THE INVENTION

Nickel electroplating has been conventionally used for electronic materials because of its good properties of the resulting film such as anti-corrosion and electrical conductivity. Conventional nickel plating composition comprises boric acid as a pH buffer, to maintain the pH value of a nickel plating bath around 3 to 5. However, boric acid is considered as one of environmental harmful chemicals. For example, boric acid is listed as a controlled chemical of the Water Pollution Prevention Act in Japan, as well as a potential chemical of Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) in the EU. Therefore, boric acid free nickel electroplating composition is desired.

Some boric acid free nickel electroplating baths are disclosed, for example, JP2012126951A, JP2004265253A, JP2001172790A and JP2010267208A. However, the nickel electroplating compositions disclosed in these patent documents include organic carboxylic acids such as citric acid or gluconic acid, or other organic compounds as pH buffer.

It is known that the pH value of a boric acid free nickel plating bath comprising organic carboxylic acids is easily increased (i.e. the bath pH is not stable) during the plating process using the bath. In addition, when these organic carboxylic acids are included in a nickel plating composition, internal stress of nickel plating film formed from the nickel plating composition is increased. Therefore, boric acid free nickel electroplating composition with good bath pH stability and good nickel plating film properties is still desired.

SUMMARY OF THE INVENTION

Inventors of this invention found that when sulfamic acid or its salt is used in a nickel electroplating composition instead of boric acid, a nickel electroplating bath using the composition has good pH stability. In addition, the nickel plating film formed from the nickel electroplating composition with sulfamic acid or its salt has similar internal stress to the one formed from the conventional nickel electroplating compositions comprising boric acid. It means that the nickel plating film formed from the composition comprising sulfamic acid or its salt can be an alternative of the conventional nickel electroplating compositions used for electronic materials.

Therefore, one embodiment of the invention is a nickel electroplating composition comprising 0.8 to 2.8 mol/L of nickel ions, 0.06 to 1.5 mol/L of halogen ions and 1.6 to 5.1 mol/L of sulfamate ions; the total amount as mol/L of sulfamate ions and halogen ions are larger than the twice of mol/L of nickel ions; the nickel electroplating composition has a pH value of 3 to 5 and the nickel electroplating composition is substantially free of boric acid and organic carboxylic acids.

Another embodiment of the invention is a nickel electroplating composition formed from 100 g/L to 650 g/L of nickel sulfamate, 2 g/L to 100 g/L of nickel halide, 5 g/L to 130 g/L of sulfamic compound selected from sulfamic acid, sodium sulfamate, potassium sulfamate and ammonium sulfamate, water and optionally surfactants, pH adjustor, wetting agent and grain refiner; the nickel electroplating composition has a pH value of 3 to 5 and the nickel electroplating composition is substantially free of boric acid and organic carboxylic acids.

A further embodiment of the invention is a nickel electroplating composition consisting of one or more sources of nickel ions, one or more sources of halogen ions, one or more sources of sulfamate ions selected from sulfamic acid, sodium sulfamate, potassium sulfamate and ammonium sulfamate, water and one or more optional compounds selected from surfactants, pH adjustor, wetting agent and grain refiner; the nickel electroplating composition having a pH value of 3 to 5 and wherein the nickel electroplating composition is substantially free of boric acid and organic carboxylic acids.

Further, the invention relates to a method for electroplating a nickel layer on a semiconductor wafer comprising: providing a semiconductor wafer comprising a plurality of conductive bonding features, contacting the semiconductor wafer with the any composition disclosed above and applying sufficient current density to deposit a nickel layer on the conductive bonding features.

Further, the invention relates to a nickel under bump metal formed from the composition any of disclosed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron micrograph (SEM) showing a nickel under bump metal obtained by Example 2.

FIG. 2 is a SEM showing a nickel under bump metal obtained by Example 3.

FIG. 3 is a bath pH stability test result for Inventive Example 1, Comparative Examples 1 to 3.

DETAILED DESCRIPTION OF THE INVENTION

As used throughout this specification, the following abbreviations shall have the following meanings unless the context clearly indicates otherwise: ° C.=degrees Centigrade(s); g=gram(s); mg=milligram(s); L=liter(s); ml=mL=milliliter(s); cm=centimeter(s); mm=millimeter(s); μm=microns=micrometer(s); Å=angstrom; A/dm²=ASD=ampere per square decimeter; AH/L=ampere by hour per litter; and ±=plus or minus. The terms “depositing” and “plating” are used interchangeably throughout this specification. All percentages are by weight, unless otherwise noted. All numerical ranges are inclusive and combinable in any order, except where it is logical that such numerical ranges are constrained to add up to 100%.

The nickel electroplating composition of the invention is substantially free of boric acid and organic carboxylic acids, and comprises sulfamic acid or salt thereof to maintain bath pH value from 3 to 5. Preferably, the nickel electroplating composition of the invention is free of boric acid and organic carboxylic acids, and comprises sulfamic acid or salt thereof to maintain bath pH value from 3 to 5.

In one aspect of the invention, the nickel electroplating composition can be formed from nickel sulfamate, nickel halide, sulfamic compound, water and optionally additives used in traditional nickel electroplating compositions.

Commercially available nickel sulfamate can be used. The amount of nickel sulfamate in the nickel electroplating composition is from 100 to 650 g/L, preferably from 200 to 500 g/L. Nickel sulfamate forms nickel ion and sulfamate ion in a nickel electroplating composition.

Examples of nickel halide include nickel chloride and nickel bromide. Commercially available nickel halide can be used. Preferably, nickel halide is nickel chloride. The amount of nickel halide in the nickel electroplating composition is from 2 to 100 g/L, preferably from 5 to 50 g/L. When nickel halide is nickel chloride, its preferable amount is from 4 to 20 g/L. Nickel halide forms nickel ion and halogen ion in a nickel electroplating composition. Halogen ion helps dissolution of nickel anode.

Sulfamic compound is a compound which forms sulfamate ions in an aqueous solution. Sulfamic compound comprises sulfamic acid and sulfamate salts such as sodium sulfamate, potassium sulfamate and ammonium sulfamate. The sulfamate salts are water soluble. Preferably, sulfamic compound is selected from sulfamic acid, sodium sulfamate, potassium sulfamate and ammonium sulfamate. The combination of two or more of sulfamic compounds can be used. Preferably, the sulfamic compound is a mixture of sulfamic acid and sulfamate salts. When the sulfamic compound is a mixture of sulfamic acid and sulfamate salts, the molar ratio of sulfamic acid and sulfamate salts is from 1:3 to 1:300. More preferably, the molar ratio of sulfamic acid and sulfamate salts is from 1:5 to 1:200.

The amount of sulfamic compound in the nickel electroplating composition is 5 g/L or more, preferably 10 g/L or more, more preferably 20 g/L or more. At the same time, the amount of sulfamic compound in the nickel electroplating composition is 600 g/L or less, preferably 300 g/L or less, more preferably 200 g/L or less, the most preferably 130 g/L or less. Sulfamic compound forms sulfamate ion and counter cation in a nickel electroplating composition.

The nickel electroplating composition of the invention comprises larger amount of sulfamate ions than a conventional nickel electroplating composition. Sulfamate ions in the nickel electroplating composition are came from nickel sulfamate and sulfamate compound. Preferably, the nickel electroplating composition comprises 0.5 to 3.0 mol/L of nickel ions, 0.03 to 2.0 mol/L of halogen ions and 1.0 to 6.5 mol/L of sulfamate ions. More preferably, the nickel electroplating composition of the invention comprises 0.8 to 2.8 mol/L of nickel ions, 0.06 to 1.5 mol/L of halogen ions and 1.6 to 5.1 mol/L of sulfamate ions. The total amount as mol/L of sulfamate ions and chloride ions are larger than twice the amount (in mol/L) of nickel ions.

Normally, the pH value of a plating bath will gradually increase during its operation. When the pH value of the nickel electroplating bath is lower than 3, the depositing speed of nickel metal would be decreased. When the pH value of the nickel electroplating bath is higher than 5, the internal stress of deposited nickel metal would be increased. Therefore, it is important to maintain the pH value of a nickel electroplating bath between 3 and 5.

Inventors of this invention found that the specific amount of sulfamate ions work like a pH buffer in a nickel electroplating bath, to maintain the pH value of a nickel plating bath between 3 to 5. Not being bound by theory, but it is considered that sulfamate ions act like a pH buffer by controlling generation of hydrogen in a bath, same as a boric acid in a bath. Therefore, the nickel electroplating composition of the invention has high pH-stability without including boric acid or organic carboxylic acids.

The nickel electroplating composition optionally comprises surfactant, pH adjustor, wetting agent and grain refiner. Such optional additives are well-known to those skills in the art. However, the additive of the nickel electroplating composition does not include organic carboxylic acids and boric acid, because the nickel electroplating composition of the invention is substantially free of boric acid and organic carboxylic acids, and preferably free of boric acid and organic carboxylic acids.

The solvent of the nickel electroplating composition is normally water. Tap water, deionized water, or distilled water can be used.

One aspect of the invention is a nickel electroplating composition consisting of one or more sources of nickel ions, one or more sources of halogen ions, one or more sources of sulfamate ions selected from sulfamic acid, sodium sulfamate, potassium sulfamate and ammonium sulfamate, water and one or more optional compounds selected from surfactants, pH adjustor, wetting agent and grain refiner. As disclosed above, the nickel electroplating composition has a pH value of 3 to 5 and the nickel electroplating composition is substantially free of boric acid and organic carboxylic acids. Preferably, the source of nickel ions is nickel sulfamate. Preferably, the concentration of one or more sources of sulfamate ions are in amounts of from 5 to 130 g/L. Such nickel electroplating composition is free of boric acid and organic carboxylic acids.

The nickel electroplating composition of the invention is useful for electronic materials. One aspect of the invention relates to a method of electroplating a nickel layer on a semiconductor wafer comprising: providing a semiconductor wafer comprising a plurality of conductive bonding features, contacting the semiconductor wafer with any composition disclosed above and applying sufficient current density to deposit a nickel layer on the conductive bonding features.

Examples of the semiconductor wafer include, but are not limited to, silicon wafer, glass and organic substrates. Conductive bonding features are normally formed by the following steps: forming a conductive layer on the surface of the semiconductor wafer, forming a resist layer on the conductive layer of the semiconductor wafer, then removing at least a part of the resist layer to make openings on the conductive layer. Copper is normally used as the conductive layer. The conductive layer can be formed any known methods, such as sputtering or electroless metal plating.

The semiconductor wafer with conductive layer is contacted with the composition disclosed above by any known methods to deposit nickel on the conductive layer of the semiconductor wafer. Normally, the semiconductor wafer is immersed in the nickel plating solution and current applied.

Nickel metal can be used as an anode, but an insoluble electrode such as a platinum-plated titanium plate can be used in some cases. The current densities are in the range of from 0.5 to 40 A/dm², preferably from 5 to 20 A/dm². The temperature of the nickel plating composition is basically from 10 to 80° C., preferably 30 to 65° C. Plating time depends on current density and required plating thickness. For example, if current density is 1 A/dm² and the required thickness of Ni layer is 3 micrometer, the plating time is about 15 minutes.

The nickel layer formed from the method of the present invention has 40 MPa or less of internal stress. More preferably, the internal stress is 30 MPa or less, and the most preferably, the internal stress is 25 MPa or less. Internal stress was measured by deposit stress analyzer. Since the internal stress formed from a conventional nickel electroplating bath comprising boric acid is around 10 to 40 MPa, the nickel electroplating composition of the invention provides nickel deposition with similar internal stress with the conventional bath.

The nickel plating composition of the invention can be used to form an under layer of gold plating and a barrier layer of copper surface. The nickel plating composition of the invention is also useful to form an under bump metal (UBM). UBM is a protection buffer layer between a seed metal (around 2000 Å of copper) and a solder. Also, the nickel plating composition of the invention can be used to form a bump (pillar) on a substrate to connect between a substrate and an electronic component electrically.

Examples

Inventive Examples 1-3 and Comparative Examples 1-3 Silicon wafer provided by IMAT Co. was used as a test sample (Test Sample). Test sample is 60 mm×50 mm size of silicon wafer which has titanium layer (under layer) and copper layer (upper layer) on the surface of the silicon wafer, then resist layer was formed on the copper layer and ten holes with 75 micrometers of diameters were formed through the resist layer. Titanium layer was formed by sputtering of 1,000 Å of titanium particles while copper layer was formed by sputtering of 3,000 Å of copper particles. Test Sample was immersed in a nickel plating solution (disclosed below) and electroplated. Anode was nickel metal. Current density was 6 A/dm², and temperature of the nickel plating solution was 60° C. The targeted plating thickness was 3 μm. Then Test Sample was washed with DI water. After that, the resist was removed by dipping in Shipley BPR stripper (Available from Rohm and has Electronic Materials, Marlborough, Mass., USA) at 60° C. for 5 minutes. Ten nickel depositions (nickel under bump metal) were formed in the holes on Test Sample. The surface of the nickel depositions were observed by SEM. The internal stress of the nickel depositions were measured by the deposit stress analyzer provided by Electrochemical co. ltd. The bath pH stability was checked and the result is shown in FIG. 3.

Nickel Plating Solution(s)

Nickel sulfamate 4-hydrate: 500 g/L (90 g/L as Nickel metal)

Nickel chloride 6-hydrate: 20 g/L (6 g/L as chloride ion)

pH buffer: disclosed in Tables 1 and 2

remainder: distilled water

pH was adjusted to around 4 by adding NaOH or H₃NSO₃

For pH buffer, the compounds written in Tables 1 and 2 were used.

TABLE 1 Inventive Examples pH buffer 1 2 3 Sulfamic acid 20 g/L 1 g/L Sodium 20 g/L 125 g/L sulfamate Internal stress 18 MPa 20 MPA 20 MPa

TABLE 2 Comparative Examples pH buffer 1 2 3 Malic acid 20 g/L Maleic acid 20 g/L Boric acid 40 g/L Internal stress 60 MPa 160 MPa 20 MPa

A SEM of a UBM obtained by Example 2 is shown in FIG. 1 (magnification is 1500 times) and SEM photograph of a UBM obtained by Example 3 is shown in FIG. 2 (magnification is 1000 times). 

What is claimed is:
 1. A nickel electroplating composition comprising 0.8 to 2.8 mol/L of nickel ions, 0.06 to 1.5 mol/L of halogen ions and 1.6 to 5.1 mol/L of sulfamate ions; the total amount as mol/L of sulfamate ions and halogen ions is greater than twice the total amount in mol/L of nickel ions; the nickel electroplating composition having a pH value of 3 to 5; wherein the nickel electroplating composition is substantially free of boric acid and organic carboxylic acids.
 2. A nickel electroplating composition consisting of one or more sources of nickel ions, one or more sources of halogen ions, one or more sources of sulfamate ions selected from the group consisting of sulfamic acid, sodium sulfamate, potassium sulfamate and ammonium sulfamate, water and one or more optional compounds selected from the group consisting of surfactants, pH adjustors, wetting agents and grain refiners; the nickel electroplating composition having a pH value of 3 to 5; wherein the nickel electroplating composition is substantially free of boric acid and organic carboxylic acids.
 3. The nickel electroplating composition of claim 2, wherein the source of nickel ions is nickel sulfamate.
 4. The nickel electroplating composition of claim 3 wherein the nickel sulfamate is present in an amount of 100 g/L to 650 g/L.
 5. The nickel electroplating composition of claim 2 wherein the nickel halide is present in an amount of 2 g/L to 100 g/L.
 6. The nickel electroplating composition of claim 5 the nickel halide is nickel chloride.
 7. The nickel electroplating composition of claim 2 wherein the one or more sources of sulfamate ions are present in an amount of 5 g/L to 130 g/L.
 8. A method of electroplating a nickel layer on a semiconductor wafer comprising: providing a semiconductor wafer comprising a plurality of conductive bonding features; contacting the semiconductor wafer with the nickel electroplating composition of claim 1; and applying sufficient current density to deposit a nickel layer on the conductive bonding features.
 9. A method of electroplating a nickel layer on a semiconductor wafer comprising: providing a semiconductor wafer comprising a plurality of conductive bonding features; contacting the semiconductor wafer with the nickel electroplating composition of claim 2; and applying sufficient current density to deposit a nickel layer on the conductive bonding features. 